This whole thread would be an excellent justification for following 3.9 with 4.0. It's as near as we ever want to get to a breaking change, and a major version number would indicate the need to review. If increasing strictness of escape code interpretation in string literals is the only incompatibility there would surely be general delight.
Kind regards, Steve Holden
I rather doubt that allowing breaking changes into a Python 4.0 would end up with this as the only proposed incompatibility. Once word got out, a flood of incompat requests would probably get raised. I personally have a change I'd like made to doctest (https://bugs.python.org/issue36714), and I know of another in argparse (https://bugs.python.org/issue33109) that I'm personally neutral on but that others have stronger feelings about.